A screw extruder is disclosed in DE 29 24 269 A, in which displaceable pressure plungers are arranged in the extruder housing normal to the extruder screw to control the flow cross section for the plastic material. This has substantial influence on the most important parameters in regard to the material being processed, namely pressure and temperature. Other factors affecting the processing are the heat supplied to the extruder housing and the viscosity of the material being processed. Control of the flow cross section is usually effected by a so-called on-line quality regulator, by which regulation of the quality of the product is achieved as a function of measured results of the product. It has been found that even small changes in the flow cross section affect the quality of the product.
Especially in the case of an arrangement with several throttle plungers, it is therefore a disadvantage if the flow cross section formed by several throttle surfaces can, depending on the magnitude of the gap, produce a gap of varying width over the circumference of the extruder screw. This is especially the case when a relatively large change in the size of the gap is contemplated.
However, eccentricity during operation, which is caused by the weight of the extruder screw, can lead in the long term, to undesirable changes in the throttling system especially in the case of large volume extruders. In addition, greatly varying radial forces, which produce bending moments on the shaft of the extruder screw also act during steady state operation because of the varying gap widths over the circumference of the extruder screw.